In recent years, worldwide demand for wireless cellular communications has increased dramatically. Radiotelephones manufactured to meet this burgeoning demand must adhere to standards such as the Global System for Mobile Communications (GSM) standard. Another standard, the Digital Cellular System (DCS) standard, is based on GSM, but is directed towards higher cell density and lower power. A third standard, Personal Communications Services (PCS) is a “catch all” for many digital cellular systems, including GSM, operating in North America. These standards all require precise output power control over a large dynamic range in order to prevent a transmitter located in one cell from interfering with the reception of transmissions from other transmitters in neighboring cells.
A key component common to all radiotelephones is a radio frequency (RF) power amplifier (PA). In modern digital radiotelephones, PAs receive as input a modulated RF carrier. The radio frequency carrier is what “carries” digital information such as digitized voice or data to a cellular base station. Before reaching the PA, the RF carrier is too weak to be received by a cellular base station. Therefore, it is the function of the PA to boost the power of the RF carrier to a level sufficient for reception by a cellular base station.
In GSM radiotelephones, the adjustable power control signal must comply with a specification known as a “burst mask.” The burst mask specifies the rise time, fall time, duration, and power levels associated with the adjustable power control signal. The GSM signal consists of eight equal time slots. Each time slot must conform to the burst mask specification. The output of an integrator circuit may be used to control the ramp-up time and ramp-down time of a PA control signal that is responsive to a dynamic baseband signal known as VRAMP. The amplitude of VRAMP dictates that the output power of the PA must conform to the shape of the burst mask.
A problem manifests itself in the prior art due to undesirable switching transients that occur when the up and down ramp of the burst is not smooth or changes shape nonlinearly. These switching transients also occur if the control slope of the power amplifier has an inflection point within the output range, or if the control slope is very steep. In particular, this problem will occur when the integrator circuit output attempts to drive the PA beyond its maximum output power capability, as shown in FIG. 1. In this situation, the integrator will ramp up to a maximum possible voltage level such as a power supply voltage, while the output power of the PA will remain at a saturated level.
When the burst is completed, VRAMP needs to ramp down. However, the integrator circuit output voltage will have to first fall from the maximum possible voltage level, as shown in FIG. 2. During the time it takes for the integrator circuit output voltage to begin to fall, the PA output power will not immediately follow VRAMP down. However, once VRAMP decreases to a voltage level in which the PA is once again controllable, the integrator circuit output voltage must “catch up” with VRAMP. As a result, there will be a sharp drop in the PA's output power, as shown in FIG. 3. As illustrated in FIG. 4, this sharp drop in the PA's output power typically results in failure of the European Telecommunications Standards Institute (ETSI) switching spectrum specification. Notice that both the −400 kHz signal and the +400 kHz signal exceed the ETSI limit as VRAMP descends.
Thus, there remains a need to provide a circuit and methodology for controlling the saturation levels of power amplifiers to prevent switching transients and maintain desirable switching spectrums for the power amplifiers' outputs.